Oil Bypass Question

[Anglocanadian]

I recently acquired a 2.7s engine 7R 911/85 case,the oil pressure pistons are the later type with no holes but the case Doesn't appear to have the bypass mod done(see photo)



Should I change the pistons to the older type ?

[Anglocanadian]

A google search bought up "Porsche implemented the oil bypass modification in 1976".
I don't know how the factory mod works so can anyone tell from the picture if this has the bypass ? It obviously does not have the plug and cross drill.

[Henry Schmidt]

This case is not modified. The modification consists of plugging that hole (threaded plug, pressed in allow plug or welding) then the oil is diverted to the supply side of the oil pump via a diagonal passage bored in the case.

[stownsen914]

Make sure you either do the bypass mod, or put the old pistons back. The parts for the two setups are not interchangeable. They will appear to fit, but word has it that the engine won't develop proper oil pressure.

[Anglocanadian]

Originally Posted by stownsen914

Make sure you either do the bypass mod, or put the old pistons back. The parts for the two setups are not interchangeable.

That's my problem the engine was fitted with the plain newer pressure pistons , I think I should replace them with the older style with the holes if I'm not doing the cross drill/plug mod.

[Anglocanadian]

Originally Posted by Henry Schmidt

This case is not modified.

I thought the factory modded cases might use a different method of diversion.

[Henry Schmidt]

Originally Posted by Anglocanadian

I thought the factory modded cases might use a different method of diversion.

Simply put, the oil modification diverted oil in by-pass mode from dumping straight into the oil pump supply passage instead of into the engine sump.
No matter if you do the modification or not, the pistons with holes in them should be replace, Update springs, plugs and pistons should be installed in every early case, 65-98.

[Anglocanadian]

A closer inspection of the case shows that there is a cross drilling that connects to the lower hole.



Did the factory tap the hole or use a bung that could have possibly fallen out as this engines history is unknown to me ?

[stownsen914]

Here's a thread that shows how the bypass mod is done, and how it should look afterward if the mod was done to your case. At a glance I don't see the hole plugged on your case that I would expect.

https://forums.pelicanparts.com/911-engine-rebuilding-forum/439287-oil-bypass-mods-help-please.html

[Anglocanadian]

I have previously done the mod on my 2.2 engine but the layout on this 2.7 is not the same. Daylight is visible when looking down from the feed hole, there is no angled cross drilling but it does have cross drilled oil ways from the side and front of the case which intersect to create the connection.
I decided to go ahead with the plugging but found it impossible to tap straight due to the lack of space , I don't have access to a lathe so used a 29/64" freeze plug and epoxy.





I'm still puzzled by the slotted lower hole which you can see below the plug.

[stownsen914]

I don't have a crankcase in front of me to confirm, but I think the small lower hole is there to give oil that seeps past the piston a place to exit, or else there'd eventually be hydraulic lock and prevent the piston from moving.